Briefly, it consists of molecules with the following constraints:
1. Well established heat of formation. This allows accurate thermochemical comparisons. However we have started adding calculations on ions, very few of which have thermochemical data.
2. Mostly atoms with atomic number less than 18 (). Since release 11 we've added a few atoms from the K through Kr row (Ca, Ti, Cu, Zn, Se, and Br).
3. Mostly less than six heavy atoms and twenty or fewer total atoms. The small size facilitates the use of common molecular orbital programs. We are adding some substituted benzenes (for example C₆H₅CH₃ toluene, C₆H₅CHCH₂ styrene, C₆H₅NH₂ aniline)

There are currently 1272 species in the CCCBDB, so not every molecule that meets the above criteria is included.